Starting making dart2js strong mode clean.

pkg/compiler/lib/src/native/behavior.dart

Index: pkg/compiler/lib/src/native/behavior.dart
diff --git a/pkg/compiler/lib/src/native/behavior.dart b/pkg/compiler/lib/src/native/behavior.dart
index 785f4cbe76fe1516a0f6116613ebd14ff12aed9b..3c950cc9e54d2ec935f0faf427feefbc786ebd36 100644
--- a/pkg/compiler/lib/src/native/behavior.dart
+++ b/pkg/compiler/lib/src/native/behavior.dart
@@ -523,14 +523,14 @@ class NativeBehavior {
       return new NativeBehavior();
     }
 
-    var specArgument = argNodes.head;
+    dynamic specArgument = argNodes.head;
     if (specArgument is! StringNode || specArgument.isInterpolation) {
       reporter.reportErrorMessage(
           specArgument, MessageKind.WRONG_ARGUMENT_FOR_JS_FIRST);
       return new NativeBehavior();
     }
 
-    var codeArgument = argNodes.tail.head;
+    dynamic codeArgument = argNodes.tail.head;
     if (codeArgument is! StringNode || codeArgument.isInterpolation) {
       reporter.reportErrorMessage(
           codeArgument, MessageKind.WRONG_ARGUMENT_FOR_JS_SECOND);